The present invention is directed to bicycle transmissions and, more particularly, to an automatic bicycle hub transmission which shifts gears in accordance with centrifugal force created by rotation of the hub.
U.S. Pat. No. 3,603,178 shows a bicycle equipped with an internal shifter hub wherein the gear ratio may be shifted automatically in accordance with centrifugal force created by the rotating wheel. Such an internal shifter hub comprises a hub axle, a driver capable of rotating about the hub axle, a hub shell, a planet gear mechanism for changing the speed of rotation of the driver and transmitting the result to the hub shell, a clutch mechanism for transmitting the output of the planet gear mechanism to the hub shell or stopping such transmission, a clutch-switching mechanism for switching the clutch mechanism by centrifugal force, and a support sleeve disposed between the planet gear mechanism and the hub shell.
The planet gear mechanism comprises an inner-tooth gear, a sun gear, a plurality of planet gears for meshing with the inner-tooth gear and the sun gear, and a planet gear carrier for supporting the plurality of planet gears. The planet gear carrier is formed integrally with the driver. The support sleeve is rotatably supported on the hub axle and is joined by a serrated joint with the planet gear carrier. A one-way clutch is mounted between the support sleeve and the hub shell.
The clutch mechanism has a ratchet pawl disposed between the inner-tooth gear and the hub shell, links the two when the ratchet pawl is in an engaged position, and disengages the two when the ratchet pawl is in a disengaged position. The hub shell is driven in an upshift mode via the planet gear mechanism when the clutch mechanism is in the engaged position, and the hub shell is driven while being directly linked to the driver when the ratchet pawl is in the disengaged position.
The clutch-switching mechanism comprises a weight member that swings by centrifugal force and a control member that rotates in response to swinging of the weight member. The weight member and control member are mounted on a weight support, itself mounted rotatably on the support sleeve. The weight support is nonrotatably linked to the inner-tooth gear.
The control member uses circular movement to switch the ratchet pawl of the clutch mechanism between the engaged position and the disengaged position. An upright control pin for interlocking with the control member is provided in the vicinity of the center about which the weight member is swung. The control member is provided with a slit for interlocking with the control pin and with a control hole for controlling the clutch pawl. The interlocking of the control pin and the slit converts the rocking movement of the weight member into the circular movement of the control member.
In such a conventional internal shifter hub, the ratchet pawl of the clutch mechanism is kept by the control hole of the control member in a position in which the pawl is separated from the hub shell until a rotational speed promoting the outward swinging of the weight member is reached. During this time, rotation transmitted from the sprocket to the driver is transmitted from the carrier to the hub shell via the support sleeve and the one-way clutch, and the hub shell is driven in a direct-coupled mode. When a certain rotational speed is achieved and the weight member has swung outward, the control pin turns while sliding in relation to the slit and causes the control member to rotate. When this happens, the ratchet pawl, controlled by the control hole, rises to the engaged position in which it is interlocked with the hub shell. In this state, the rotation that has been transmitted from the sprocket to the driver is upshifted and outputted from the carrier to the inner-tooth gear via the planet gears, and is further transmitted to the hub shell via the clutch mechanism, thus driving the hub shell in an upshifted mode.
In this arrangement wherein the clutch mechanism is switched and the speed is changed using centrifugal force, the extent of rocking of the weight member is very small because the weight member has a limited housing space. Additionally, the control pin has a short revolution path in relation to the rocking of the weight member because the control pin is disposed in the vicinity of the center of rocking of the weight support. The control member is therefore unable to rotate over significant distances, making it necessary that the ratchet pawl be switched between the engaged position and the disengaged position by a very small amount of circular movement.
Another drawback is that large centrifugal forces are difficult to obtain because of the limited housing space for the weight member. It is therefore necessary to increase the efficiency with which the rocking movement is converted to a circular movement in the clutch-switching mechanism. With the conventional arrangement described above, however, the efficiency with which the rocking movement is converted to a circular movement is low, the circular movement of the control member is impeded in relation to the rocking of the weight member, and smooth shifting is difficult to achieve because the control pin interlocked with the slit must slide along the slit to convert the rocking movement of the weight member into the rotational movement of the control member.